This invention relates to an air separation method and apparatus. More particularly, the present invention relates to an air separation method and apparatus which are suitable for separating air in a single rectification column with a high recovery rate of oxygen with small power consumption per unit of oxygen.
In general, power consumption per unit of oxygen, that is, the energy supply required for the separation of a unit of the product, as well as a degree of reduction in size and structural simplicity of components used, are factors considered for evaluating the efficiency of air separators. The power consumption per unit of oxygen is a function between the recovery rate of separated gas products, for example, oxygen gas, and the discharge/intake pressure by a feed air compressor, that is, the higher the recovery rate of oxygen gas is or the lower the discharge pressure in the feed air compressor is, the smaller the power consumption per unit of oxygen. In other words, provided the cases are equivalent in compact size and structural simplicity of component equipment, the smaller the power consumption per unit of oxygen, the higher the efficiency of the air separators.
In accordance with the prior art, especially in its early stages, air separation has been effected by use of an air separator using a single rectification column in which a large number of plates are provided in the direction of its height, and a condenser is provided at the bottom immersed in liquid oxygen. Feed air that is compressed by an air compressor and that is cooled by a heat exchanger for cooling the feed air is subjected to a heat exchange in the condenser with the liquid oxygen therearound, is condensed and liquefied thereby and is thereafter supplied into the single rectification column from its upper portion as a reflux liquid. At the same time, the liquid oxygen around the condenser is vaporized and is converted into an upwardly flowing vapor. This upwardly flowing vapor comes into vapor-liquid contact with the reflux liquid on the plates and thus rectification proceeds. As a result, a gas rich in nitrogen (nitrogen-rich gas) is withdrawn from the top of the single rectification column, and gaseous oxygen having a high purity is taken from the bottom of the column.
In accordance with the air separation method and apparatus of the type described above, the size reduction and structural simplicity factors are superior due to the air separation performed in a single rectification column; however, the reflux liquid supplied from the upper portion of the single rectification column has the composition of air. Accordingly, though pure gaseous oxygen can be withdrawn from the bottom of the column, the upper limit of the nitrogen concentration of the gas withdrawn from the top of the column is 93%, and the recovery rate of the oxygen remains at a low level. Hence, there is still the problem that the power consumption per unit of oxygen is high.
For this reason, air separation is carried out nowadays by an air separator using a double rectification column consisting of a lower column incorporating a large number of plates aligned in the direction of its height and operating at a high pressure, and an upper column thermally coupled to the lower column by a reboiler-condenser, incorporating a large number of plates in the direction of its height and operating at a low pressure. Feed air that is compressed by an air compressor and cooled by a heat exchanger is introduced as an upwardly flowing vapor into the lower portion of the lower column. This upwardly flowing vapor is condensed and liquefied by the reboiler-condenser into a reflux liquid which flows down through the lower column. During this time, this reflux liquid comes into vapor-liquid contact with the upwardly flowing vapor on the plates, a preparatory rectification occurs so that liquid nitrogen having a high purity can be obtained at the top of the lower column and liquefied air rich in oxygen (about 38% O.sub.2) can be obtained at the bottom. The pure liquid nitrogen and the liquefied air rich in oxygen are withdrawn from the lower column at their respective positions and, after being expanded to the pressure of the upper column in expansion valves, they are supplied to the upper column from its top and its intermediate portion as a reflux liquid for the upper column.
Liquid oxygen at the bottom of the upper column is heated by the nitrogen at the top of the lower column in the reboiler-condenser and is vaporized into an upwardly flowing vapor in the upper column. This upwardly flowing vapor comes into vapor-liquid contact with the reflux liquid on the plates so that pure gaseous nitrogen can be withdrawn from the top of the upper column, pure gaseous oxygen from the bottom, and waste gas rich in nitrogen (about 97% N.sub.2), from an intermediate portion of the column.
This air separation method and apparatus provides a greatly improved recovery rate of oxygen an a reduced power consumption per unit of oxygen; however apparatus is complicated in construction and has a relatively large size since the rectifying column consists of upper and lower columns, thereby adversely affecting the size reduction and structural simplicity factors.